Field of the Invention
The present invention provides cylindrical superconducting magnet coil structures, for example as used in Magnetic Resonance Imaging (MRI) systems. Such magnets must be cooled to below the transition temperature of the superconducting wire used which requires the superconducting magnet coil structure to be placed in a cryostat to isolate it from ambient temperature.
Description of the Prior Art
FIG. 1 shows an example conventional arrangement of a cryostat including a cryogen vessel 12. A superconducting magnet comprises a coil structure 10 within cryogen vessel 12, itself retained within an outer vacuum chamber (OVC) 14. One or more thermal radiation shields 16 are provided in a vacuum space between the cryogen vessel 12 and the outer vacuum chamber 14. In some known arrangements, a refrigerator 17 is mounted in a refrigerator sock 15 located in a turret 18 provided for the purpose, towards the side of the cryostat. Alternatively, a refrigerator 17 may be located within access turret 19, which retains access neck (vent tube) 20 mounted at the top of the cryostat. The refrigerator 17 provides active refrigeration to cool cryogen gas within the cryogen vessel 12, in some arrangements by recondensing it into a liquid. The refrigerator 17 may also serve to cool the radiation shield 16. As illustrated in FIG. 1, the refrigerator 17 may be a two-stage refrigerator. A first cooling stage is thermally linked to the radiation shield 16, and provides cooling to a first temperature, typically in the region of 80-100K. A second cooling stage provides cooling of the cryogen gas to a much lower temperature, typically in the region of 4-10K.